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Growth of high quality mercurous halide single crystals by physical vapor transport method for AOM and radiation detection applications
Journal of Crystal Growth 450 (2016) 96–102
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Growth of high quality mercurous halide single crystals by physical vapor transport method for AOM and radiation detection applications Priyanthi M. Amarasinghe a,n, Joo-Soo Kim a, Henry Chen a, Sudhir Trivedi a, Syed B. Qadri b, Jolanta Soos a, Mark Diestler a, Dajie Zhang a, Neelam Gupta c, Janet L. Jensen d, James Jensen d a
Brimrose Technology Corporation, Sparks, MD, USA U.S. Naval Research Laboratory, Washington D.C., USA c U.S. Army Research Laboratory, Adelphi, MD, USA d U.S. Army Edgewood Chemical Biological Center, Aberdeen Proving Ground, MD, USA b
art ic l e i nf o
a b s t r a c t
Article history: Received 5 January 2016 Received in revised form 9 June 2016 Accepted 16 June 2016 Communicated by Klaus Jacobs Available online 18 June 2016
Single crystals of mercurous halide were grown by physical vapor transport method (PVT). The orientation and the crystalline quality of the grown crystals were determined using high resolution x-ray diffraction (HRXRD) technique. The full width at half maximum (FWHM) of the grown mercurous bromide crystals was measured to be 0.13 degrees for (004) reflection, which is the best that has been achieved so far for PVT grown mercurous halide single crystals. The extended defects of the crystals were also analyzed using high resolution x-ray diffraction topography. Preliminary studies were carried out to evaluate the performance of the crystals on acousto-optic modulator (AOM) and gamma-ray detector applications. The results indicate the grown mercurous halide crystals are excellent materials for acousto-optic modulator device fabrication. The diffraction efficiencies of the fabricated AOM device with 1152 and 1523 nm wavelength lasers polarizing parallel to the acoustic wave were found to be 35% and 28%, respectively. The results also indicate the grown crystals are a promising material for gamma-ray detector application with a very high energy resolution of 1.86% FWHM. & 2016 Elsevier B.V. All rights reserved.
Keywords: A1. High resolution X-ray diffraction A1. X-ray topography A2. Growth from vapor A2. Single crystal growth B1. Halides B2. Acousto-optic materials
1. Introduction Acousto-optic tunable filters (AOTF's) and acousto-optic modulators (AOM's) are used in multispectral and hyperspectral applications such as the detection of targets, backgrounds, and stand-off chemical and biological agents [1–8]. Currently, tellurium dioxide (TeO2), which has a spectral transmission range of 0.35– 5.0 μm, has been the most commonly used AOTF material. However, TeO2 is not efficient in some applications such as chemical agents since infrared absorption/emission primarily occurs in these materials in the 8–12 μm wavelength region. Therefore, there is a need for technologically advanced AOTF material for 8– 12 μm wavelength region. Mercurous halides (Hg2X2), i.e. Hg2Cl2, Hg2Br2 and Hg2I2, have been of great interest as the AOTF material for fabricating AOTF-based spectral imaging systems in the long wavelength infrared (LWIR) from 8 to 12 μm spectral region. Mercurous halides can be processed at relatively low n
Corresponding author. E-mail address: [email protected] (P.M. Amarasinghe).
http://dx.doi.org/10.1016/j.jcrysgro.2016.06.025 0022-0248/& 2016 Elsevier B.V. All rights reserved.
temperatures, and have a high birefringence and relatively high transmission in the 8–12 μm spectral range. Material also possesses a high acousto-optic figure of merit (M2), which is given by,
M2 =
n6p2 ρν 3
(1)
where, n is the refractive index, p is the elastic constant, ρ is the density and the ν is the velocity of the sound in the material. Despite its high acousto-optic figure of merit for LWIR application, mercurous halides have not been extensively exploited due to lack of commercial availability and lack of standardized device processing techniques. Mercurous halides can detect gamma and potentially neutron radiation making it possible to detect two types of radiation with just one crystal material. The materials have wider bandgaps compared to the most currently available gamma ray detector materials. Due to the material's high resistivity and low leakage current, Hg2X2 makes the new technology more compatible with available microelectronics. It was seen that mercuric iodide gamma detector performance is comparable to or even better than
P.M. Amarasinghe et al. / Journal of Crystal Growth 450 (2016) 96–102
2. Experimental
CdTe/CZT [9]. Mercurous iodide (Hg2I2), which is a similar mercuric iodide (HgI2), has the same potential but with additional advantages such as being non-hygroscopic and non-toxic. In the past few decades, several attempts have been made in growing Hg2X2 single crystals [10–13]. To our knowledge, the best rocking curve of mercurous halide reported so far exhibits full width half maximum (FWHM) of 0.7802° for Hg2Br2 single crystals [13]. Growing high quality Hg2X2 crystals involves very careful growth and processing techniques, mainly due to the requirement of very low vapor pressure – starting from purification of the raw material to growing of the crystals. During the growth, the most prominent defects generated include inclusions of mercury, formation of grain boundaries and development of micro-cracks. In order to optimize the device performance, these defects need to minimize during the crystal growth. Mercurous halide crystalizes in the tetragonal phase with a space group of I4/mmm (139). The lattice parameters a and c differ in Hg2Cl2, Hg2Br2 and Hg2I2 structures due to the differences in the ionic radii of the halide atoms. Fig. 1(a) shows the relationship between the lattice parameter c [14–16] and the ionic radii [17] of the halogen (X) atoms. The lattice parameters a and c of Hg2I2 are 4.8974(9) Å and 11.649(2) Å, respectively. The atomic structure of Hg2I2 is shown in Fig. 1(b). The unit cell contains two Hg2I2 molecules. Quality of the acousto-optic crystal is the key to the high performance of the AOTF devices. The orientation of the crystal is also a crucial factor in fabricating high performance AOTF devices. X-ray diffraction (XRD) analysis provides an enormous information on the quality of the crystals as well as on the orientation of the crystals. While XRD 2θ–θ data provide the information on the composition and the orientation, the rocking curves provide the important information on the crystalline quality of the crystals. Purity of the starting feeding material is extremely important in growing high quality crystals, as even trace amount of impurity may affect the crystal quality. Therefore, the raw material needs to be thoroughly purified prior to the crystal growth. Thus, x-ray characterization can also be used to optimize the growth parameters and hence the quality of the crystals. In this study, we present (i) the growth of large crystals of Hg2X2 by PVT method and their characterization using high resolution rocking curves and x-ray topography. We also present the preliminary test results of the acousto-optic modulator (AOM) that was fabricated using the PVT grown Hg2X2 crystals. In this research, we also demonstrate the viability of the PVT-grown Hg2X2 crystals as an attractive material for radiation detection.
2.1. Crystal growth of Hg2X2 Commercially available 99.9% Hg2X2 powder which was purchased from Kyantec Inc., Louisville, KY, was used as the starting source materials. The starting source material was rigorously purified by repeated sublimation via Physical Vapor transport (PVT) under high vacuum close to 10 4 Torr, as source Hg2X2 powder may contain impurities such as, oxides, carbon, sulphur, etc. About 500 g of a given source material is loaded into the feeding chamber of the pre-cleaned and outgassed Pyrex purification ampoule. Details of the purification and growth process can be found in references [10,18–20]. The crystal growth process was controlled by the advanced PID (proportional integral derivative) controllers with adaptive tuning mode and the precise ampoule transporter system which gradually changes the temperature of the ampoule by lowering or raising it inside the heating coil. The rate at which the ampoule can be lowered raised can be set as low as 1 mm per day for smooth temperature profile along the ampoule. Details of the crystal growing furnace can be found in Ref. [18]. The design of the growth ampoule was optimized for the self-seeding technique. The temperature of the growth ampoule was increased slowly to generate a seed for single crystal. The major difference in the growth procedures of three different mercurous halide crystals is the growth temperature. The growth temperature ranges of Hg2I2, Hg2Br2 and Hg2Cl2 are 200 °F–230 °F, 300 °F–330 °F and 370 °F–400 °F, respectively. ΔT, which is the temperature difference between the source and the crystal is a critical factor for high quality crystals. It was noticed that smaller ΔT makes the crystal grow slower but higher in quality whereas larger ΔT makes the crystals grow faster but lower in quality. Once the feeding Hg2X2 material was completely sublimed and crystalized, the crystal growing process is terminated with gradual decrease of the temperature inside the growth chamber. Due to single crystalline nature of the crystals, during cooling the grown crystals completely detach from the surrounding glass of the ampoule. Several crystals of each material (mercurous iodide, mercurous bromide and mercurous chloride) have been successfully grown. The diameter of the grown crystals is 48 mm, which is more than 3 times the diameter of the best quality crystals (13 mm) that has been reported so far [13]. The crystals are 70– 80 mm in height and 400–600 g in weight. The exact height and the weight of the grown crystals depend on the amount of the
11.8 Lattice parameter c
97
Hg I
11.4
I
11.0
Br Cl
10.6 1.6
1.8 2 Ionic radii
2.2
Fig. 1. Ionic radii vs. lattice parameter c (a) and the atomic structure of the mercurous iodide (Hg2I2) unit cell.
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feeding material in the growth ampoules. XRD analysis were conducted on the naturally formed facet of the crystal as well as on the cut and polished surfaces of the grown crystals. 2.2. XRD analysis of the Hg2X2 crystals In order to determine the purity of the starting source material, XRD analysis was conducted by acquiring full 2θ–θ scan on the source material. To find out the efficiency of the purifying method,
Sample location 1 (Sublimate)
Sample location 2 (Black impurity)
Fig. 2. Schematic of the purifying ampoule showing the two different sample locations.
XRD analysis was performed on the sublimate after each purifying process. Samples from the sublimate were collected from the sample location 1 as shown in Fig. 2. In order to determine the type of impurity, the substance that remains in the feeding chamber of the purifying ampoule (sample location 2) was also collected for XRD analysis. Once the Hg2X2 crystals are fully grown, XRD analysis were performed to identify the orientation, lattice parameter c and the crystalline quality of the Hg2X2 crystals. For the analysis, a Rigaku high-resolution four-circle diffractometer with an 18-kW rotatinganode Cu source was used. In addition, high resolution x-ray diffraction (HRXRD) analysis were performed to determine the crystalline quality and the lattice parameter c of the grown crystals. During the performance of HRXRD, only the Cu Kα1 x-rays were selected using a monochromator that consists of two Ge (220) channel-cut crystals. In addition, a Ge (220) channel-cut crystal was used as the analyzer crystal. The diffractometer has a four-circle goniometer with X-, Y- and Z- translation stage in addition to ω, χ, and ϕ degrees of freedom. X-ray scans were collected from both [hk0] and [00l] reflections. In order to determine the crystalline quality, rocking curves of the [00l] reflections were
Fig. 3. Prepared 10 mm 12 mm 20 mm Hg2Br2 crystal (a), the fabricated AOM device (b) and the electrical circuit of the device (c). L1, L2 and L3 are inductors and C is a capacitor.
Fig. 4. Fabricated and tested gamma ray detector configurations consist of 12 mm 12 mm 4 mm, Hg2Br2, 12 mm 10 mm 3 mm Hg2I2 and 15 mm 15 mm 5 mm Hg2I2 substrate from left to right.
Fig. 5. Purifying ampoules before (a) and after (b) the first purifying process of Hg2I2.
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obtained from the naturally formed facet of the crystal. In addition, 2θ/ω scans were performed to determine the lattice parameter of the crystal. For device fabrication, the crystals need to be cleaved or cut and fine-polished. The XRD scans were also acquired on the polished facet and compared with the scans of the naturally formed facet to evaluate the efficiency of the polishing technique that was used. Furthermore, high resolution x-ray topographic images of the Hg2X2 crystals were taken using a double-crystal topography setup using Cu Kα1 radiation.
insulating layer, either chemically or via physical deposition methods. To evaluate detector performance, several different device configurations have been fabricated from planar to pseudohemispherical, to pixelated detectors at different thicknesses. Some of the device configurations and electrode contacts are shown in Fig. 4. The epoxy was used as both electrode and as a mean to connect the device to readout electronic circuitry using Au or Pt wires.
2.3. Acousto-optic device fabrication
3. Results and discussions
A PVT-grown Hg2Br2 crystal, size 15 mm 8 mm 3 mm, was cut and polished for fabrication of acousto-optic modulator (Fig. 3a). The optical beam is traveling along the (001) direction. The orientations of the crystal was confirmed using x-ray diffraction. Next, a pre-thinned transducer was mounted to the prepared Hg2Br2 using conductive epoxy, which also is acting as the bottom electrode. After the epoxy was cured and firmly bonded to the transducer and the substrate, a patch of epoxy was smeared on the transducer for the top electrode. Next, conductive Cu strips were attached for the electrical connections. Final part of the AO design involves electrical impedance matching network. Drawing of the electrical equivalent circuit for the fabricated device is shown in Fig. 3c. The final fabricated device for AOM application is shown in Fig. 3b.
3.1. Purification and crystal growth
2.4. Hg2X2 for gamma ray detection In preparation for gamma ray detection, samples were first cut and polished, followed by chemical etching to remove subsurface damages. Segmented pattern was created via photolithography. The inter-pixel or streets regions were passivated with an
Pictures of the purifying ampoules before and after the first purification process are shown in Fig. 5. After the first sublimation process, it was noticed that a black powdery substance (impurity) was remained in the feeding side of the ampoule. However, this substance was not further seen in any of the subsequent resublimation processes. XRD scans of the sample that was collected from the sample location 1 (sublimate) after the final purification process of Hg2I2 powder are shown in the Fig. 6. All the peaks observed corresponded to the Hg2I2 peaks as marked, indicating that the sublimate only consists of polycrystalline Hg2I2. Dark-colored substance was observed in the location no. 2 (Fig. 2) during purification as shown in Fig. 5. Literature survey indicates this material as Hg5Cl2O4 [10]. However, our XRD data did not match with the XRD pattern of Hg5Cl2O4. Thorough investigation on this residue is underway and finding will be published in an upcoming research article. The prominent cleavage planes of Hg2X2 crystals were found along (110) orientation. The crystals were then cleaved along (110) orientation and polished for XRD analysis. Photographs of cleaved and polished surfaces of the PVT-grown Hg2I2 crystal are shown in Fig. 7. XRD pattern collected along (110) orientation is shown in Fig. 8. The 2θ–θ full scan (Fig. 8-a) shows two sharp peaks that correspond to the (110) and (220) reflections. The results also displayed resolved kα1 and kα2 peaks, which can be seen only in high quality crystals. 3.2. High resolution X-Ray diffraction (HRXRD) analysis
Fig. 6. XRD pattern of the sublimate (sample location 1) after the final purification of Hg2I2.
Naturally-formed facets along (001) direction and (110) directions were observed on every PVT-grown Hg2X2 crystal. The facet along (001) direction of an Hg2I2 crystal was used in crystal quality analysis. The 2θ–ω scan and the ω scan of the naturally-formed facet of a Hg2I2 crystal from the (004) reflection was acquired using HRXRD. The measured full with at half maximum (FWHM) of the rocking curve of the Hg2I2 crystal was determined as 0.13°
Fig. 7. PVT-grown Hg2I2 crystal (a), cut and polished along the (001) plane (b), and the clean (110) cleavages (c).
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Fig. 8. XRD pattern collected along (110) orientation. (a) 2θ–θ scan (b) Resolve of kα1 and kα2 peaks on (220) reflection of Hg2I2 crystal.
(Fig. 9). In this figure, the inset is a picture of a naturally formed facet of the Hg2I2 crystal. To our knowledge, for bulk-grown single crystals, FWHM of 0.13° is the best that has been achieved so far on PVT-grown mercurous halide single crystals. The lattice parameter c of the grown Hg2I2 crystal that corresponds to 2θ at 30.77° was determined as 11.614 Å. Before making the AOM device, the cut or cleaved crystals need to be fine polished for optimal AOM performance. Smooth polishing of the bonding and the optical faces of the cut crystals is extremely important in acousto-optic applications. Cutting and rough polishing of the crystal may cause grains on the surface to become loose and reorient. In XRD these changes are observed as elevated FWHM of the rocking curve. The quality of the polished surface of the cut crystal was evaluated using HRXRD. Fig. 10 is the high resolution ω scan of a cleaved and fine polished Hg2I2 crystal from the (004) reflection. The measured full with at half maximum (FWHM) of the rocking curve was determined as 0.24°, which is very close to the measured FWHM (0.13°) of the naturally-formed
Fig. 9. High resolution rocking curve of the naturally-formed facet of Hg2I2 (004).
Fig. 10. High resolution ω scan of the cleaved and polished face of Hg2I2 crystal from the (004) reflection.
Fig. 11. Topographic image showing grain boundaries (A) and an array of threading screw dislocations (B) of the polished Hg2I2 (004) reflection.
P.M. Amarasinghe et al. / Journal of Crystal Growth 450 (2016) 96–102
facet. The results not only confirm the excellent quality of the crystal, but also indicate that the polishing method has no adverse effects on the quality of the polished surfaces. The lattice parameter c that corresponds to the 2θ at 30.70° was determined as 11.639 Å, which is very close to the lattice parameter c (11.614 Å) that was obtained from the naturally-formed facet. Topographic image of the polished Hg2I2 (004) reflection is shown in Fig. 11. In this image random line dislocations are seen as the extended defects of the crystal. These features are very consistent with the features that can be observed in the XRD topograph (not included in this paper) that was taken from a naturallyformed facet of the Hg2I2 crystal. In some locations the dislocations are leading into formation of grain boundaries (A). An array of threading screw dislocations (B) is also visible on the image.
101
3.3. AOM device testing Further testing was conducted on fabricated AOM device using 1152 nm and 1523 nm wavelength He-Ne laser beams. The first order diffraction signal of the device was clearly observed with diffraction angles of 1.2° and 2° for 1152 nm and 1523 nm laser beams, respectively. The diffraction images of the AOM with and without RF power is shown in Fig. 12. The magnitude of RF power was 1.5 W. The diffraction efficiencies of the 1152 nm laser polarizing at 45° to the acoustic wave and parallel to the acoustic wave were found to be 37% and 35%, respectively. The diffraction efficiencies of the 1523 nm wavelength laser polarizing parallel to the acoustic wave was found to be 28%.
Fig. 12. Diffraction images of the fabricated AOM (a) without the RF (b) with the RF power showing the first order diffraction.
Fig. 13. (a) Co-57 energy resolution of 2 mm Pseudo-hemispherical Hg2I2 detector at 122 keV was 1.86% FWHM. (b) Am-241 energy resolution of a 2 mm-thick Pseudo hemispherical Hg2I2 detector at 59.6 keV is 1.68% FWHM. The peaks are marked in purple.
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3.4. Gamma ray detector response To evaluate detector performance, Cobalt-57 (Co-57) spectral response was acquired using 2 mm-thick Pseudo-hemispherical Hg2I2 detector. The energy resolution of Co-57 at 122 keV was found to be 1.86% FWHM at room temperature. The energy resolution of Americium-241 (Am-241) response from a hemispherical Hg2I2 detector at 59.6 keV was found to be 1.7%. The results are shown in Fig. 13.
4. Conclusions High quality transparent single crystals of mercurous iodide (Hg2I2) of 48 mm in diameter and weighing 400–600 g were grown using unseeded PVT technique. The crystals grew without any contact with the side walls of the ampoule. The study shows the excellent crystalline quality of the grown crystals. X-ray full with at half maximum (FWHM) of the rocking curve of the natural facet was determined as 0.13°. The FWHM of the cleaved and fine polished facet was determined as 0.24°. Preliminary feasibility studies were conducted on AOM device fabrication using the grown crystals. The diffraction efficiencies with 1152 and 1523 nm wavelength lasers polarizing parallel to the acoustic wave were found to be 35% and 28%, respectively. The crystals were also used in fabricating devices for gamma ray and alpha particle detection. In this study, the energy resolutions as high as 1.86% FWHM at 122 keV and 1.7% FWHM at 59.6 keV were observed with Co-57 and Am-241, respectively at room temperature.
Acknowledgment The authors would like to thank Dave Mayers, Paul Deng, Helen
He and Julie Chen for their support on sample preparation. This research has been supported by the US Army Edgewood Chemical Biological Center, Contract number W911SR-14-C-0065 and US Army Research Laboratory, Contract numbers W911QX-06-C-0074 and W911QX-06-C-0074-P0006.
References [1] O. Cakmakci, J. Rolland, Display Technol. 2 (2006) 199–216. [2] R.J. Thompson, D.C. Aveline, L. Lundblad, L. Maleki, NASA Tech Briefs, NTR 43741, March 2007. [3] N. Gupta, SPIE Conf. Proc. 2008, 6972. [4] M.L. Nischan, R.M. Joseph, J.C. Libby, J.P. Kerekes, Lincoln Lab. J. 14 (2003) 131–144. [5] K.A. Bakeev, Process Analytical Technology, Blackwell Publishing, Oxford, 2005. [6] N. Gupta, R. Dahmani, S. Choy, Opt. Eng 41 (2002) 1033–1038. [7] N. Gupta, L. Denes, M. Gottlieb, D. Suhre, B. Kaminsky, P. Metes, Appl. Opt. 40 (2001) 6626–6632. [8] D.R. Suhre, L.H. Taylor, N.B. Singh, W.R. Rosch, SPIE Conf. Proc. 3584, 1999. [9] A.I. Skrypnyk, A.A. Zakharchenko, M.A. Khazhmuradov, E.M. Prokhorenko, V. F. Klepikov, V.V. Lytvynenko, Nucl. Phys. Investig. 60 (2013) 231–235. [10] N.B. Singh, R.H. Hopkins, R. Mazelsky, J.J. Conroy, J. Cryst. Growth 75 (1986) 173–180. [11] K. Baskarr, R. Gobmathan, Cryst. Res. Technol. 25 (1990) 747–752. [12] N.B. Singh, M. Gottlieb, A.P. Goutzoulis, R.H. Hopkins, R. Mazelsky, J. Cryst. Growth 89 (1988) 527–530. [13] D.J. Knuteson, N.B. Singh, M. Gottlieb, D. Suhre, N. Gupta, A.E. Berghmans, D. A. Kahler, B. Wagner, J. Hawkins, Opt. Eng. 46 (2007) 64001–64006. [14] E. Dorm, J. Chem. Soc. D (1971) 466–467. [15] Natl. Bur. Stand. (U. S.) Circ., 539 7, 33, 1957. [16] M. Kars, T. Roisnel, V. Dorcet, A. Rebbah, O.D.L. Carlos, Acta Crystallogr. Sec. E 68 (2012) 2. [17] R.D. Shannon, Acta Cryst. A32 (1976) 751–767. [18] J. Kim, S.B. Trivedi, J. Soos, N. Gupta, W. Palosz, J. Cryst. Growth 310 (2008) 2457–2463. [19] F. Jin, J. Kim, S. Kutcher, E. Haskovic, D. Meyers, J. Soos, S. Trivedi, N. Gupta, Proc. IEEE AIPR 2012. [20] J. Kim, S.B. Trivedi, J. Soos, N. Gupta, W. Palosz, SPIE Proc. 2007, 6661.
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Growth of high quality mercurous halide single crystals by physical vapor transport method for AOM and radiation detection applications Priyanthi M. Amarasinghe a,n, Joo-Soo Kim a, Henry Chen a, Sudhir Trivedi a, Syed B. Qadri b, Jolanta Soos a, Mark Diestler a, Dajie Zhang a, Neelam Gupta c, Janet L. Jensen d, James Jensen d a
Brimrose Technology Corporation, Sparks, MD, USA U.S. Naval Research Laboratory, Washington D.C., USA c U.S. Army Research Laboratory, Adelphi, MD, USA d U.S. Army Edgewood Chemical Biological Center, Aberdeen Proving Ground, MD, USA b
art ic l e i nf o
a b s t r a c t
Article history: Received 5 January 2016 Received in revised form 9 June 2016 Accepted 16 June 2016 Communicated by Klaus Jacobs Available online 18 June 2016
Single crystals of mercurous halide were grown by physical vapor transport method (PVT). The orientation and the crystalline quality of the grown crystals were determined using high resolution x-ray diffraction (HRXRD) technique. The full width at half maximum (FWHM) of the grown mercurous bromide crystals was measured to be 0.13 degrees for (004) reflection, which is the best that has been achieved so far for PVT grown mercurous halide single crystals. The extended defects of the crystals were also analyzed using high resolution x-ray diffraction topography. Preliminary studies were carried out to evaluate the performance of the crystals on acousto-optic modulator (AOM) and gamma-ray detector applications. The results indicate the grown mercurous halide crystals are excellent materials for acousto-optic modulator device fabrication. The diffraction efficiencies of the fabricated AOM device with 1152 and 1523 nm wavelength lasers polarizing parallel to the acoustic wave were found to be 35% and 28%, respectively. The results also indicate the grown crystals are a promising material for gamma-ray detector application with a very high energy resolution of 1.86% FWHM. & 2016 Elsevier B.V. All rights reserved.
Keywords: A1. High resolution X-ray diffraction A1. X-ray topography A2. Growth from vapor A2. Single crystal growth B1. Halides B2. Acousto-optic materials
1. Introduction Acousto-optic tunable filters (AOTF's) and acousto-optic modulators (AOM's) are used in multispectral and hyperspectral applications such as the detection of targets, backgrounds, and stand-off chemical and biological agents [1–8]. Currently, tellurium dioxide (TeO2), which has a spectral transmission range of 0.35– 5.0 μm, has been the most commonly used AOTF material. However, TeO2 is not efficient in some applications such as chemical agents since infrared absorption/emission primarily occurs in these materials in the 8–12 μm wavelength region. Therefore, there is a need for technologically advanced AOTF material for 8– 12 μm wavelength region. Mercurous halides (Hg2X2), i.e. Hg2Cl2, Hg2Br2 and Hg2I2, have been of great interest as the AOTF material for fabricating AOTF-based spectral imaging systems in the long wavelength infrared (LWIR) from 8 to 12 μm spectral region. Mercurous halides can be processed at relatively low n
Corresponding author. E-mail address: [email protected] (P.M. Amarasinghe).
http://dx.doi.org/10.1016/j.jcrysgro.2016.06.025 0022-0248/& 2016 Elsevier B.V. All rights reserved.
temperatures, and have a high birefringence and relatively high transmission in the 8–12 μm spectral range. Material also possesses a high acousto-optic figure of merit (M2), which is given by,
M2 =
n6p2 ρν 3
(1)
where, n is the refractive index, p is the elastic constant, ρ is the density and the ν is the velocity of the sound in the material. Despite its high acousto-optic figure of merit for LWIR application, mercurous halides have not been extensively exploited due to lack of commercial availability and lack of standardized device processing techniques. Mercurous halides can detect gamma and potentially neutron radiation making it possible to detect two types of radiation with just one crystal material. The materials have wider bandgaps compared to the most currently available gamma ray detector materials. Due to the material's high resistivity and low leakage current, Hg2X2 makes the new technology more compatible with available microelectronics. It was seen that mercuric iodide gamma detector performance is comparable to or even better than
P.M. Amarasinghe et al. / Journal of Crystal Growth 450 (2016) 96–102
2. Experimental
CdTe/CZT [9]. Mercurous iodide (Hg2I2), which is a similar mercuric iodide (HgI2), has the same potential but with additional advantages such as being non-hygroscopic and non-toxic. In the past few decades, several attempts have been made in growing Hg2X2 single crystals [10–13]. To our knowledge, the best rocking curve of mercurous halide reported so far exhibits full width half maximum (FWHM) of 0.7802° for Hg2Br2 single crystals [13]. Growing high quality Hg2X2 crystals involves very careful growth and processing techniques, mainly due to the requirement of very low vapor pressure – starting from purification of the raw material to growing of the crystals. During the growth, the most prominent defects generated include inclusions of mercury, formation of grain boundaries and development of micro-cracks. In order to optimize the device performance, these defects need to minimize during the crystal growth. Mercurous halide crystalizes in the tetragonal phase with a space group of I4/mmm (139). The lattice parameters a and c differ in Hg2Cl2, Hg2Br2 and Hg2I2 structures due to the differences in the ionic radii of the halide atoms. Fig. 1(a) shows the relationship between the lattice parameter c [14–16] and the ionic radii [17] of the halogen (X) atoms. The lattice parameters a and c of Hg2I2 are 4.8974(9) Å and 11.649(2) Å, respectively. The atomic structure of Hg2I2 is shown in Fig. 1(b). The unit cell contains two Hg2I2 molecules. Quality of the acousto-optic crystal is the key to the high performance of the AOTF devices. The orientation of the crystal is also a crucial factor in fabricating high performance AOTF devices. X-ray diffraction (XRD) analysis provides an enormous information on the quality of the crystals as well as on the orientation of the crystals. While XRD 2θ–θ data provide the information on the composition and the orientation, the rocking curves provide the important information on the crystalline quality of the crystals. Purity of the starting feeding material is extremely important in growing high quality crystals, as even trace amount of impurity may affect the crystal quality. Therefore, the raw material needs to be thoroughly purified prior to the crystal growth. Thus, x-ray characterization can also be used to optimize the growth parameters and hence the quality of the crystals. In this study, we present (i) the growth of large crystals of Hg2X2 by PVT method and their characterization using high resolution rocking curves and x-ray topography. We also present the preliminary test results of the acousto-optic modulator (AOM) that was fabricated using the PVT grown Hg2X2 crystals. In this research, we also demonstrate the viability of the PVT-grown Hg2X2 crystals as an attractive material for radiation detection.
2.1. Crystal growth of Hg2X2 Commercially available 99.9% Hg2X2 powder which was purchased from Kyantec Inc., Louisville, KY, was used as the starting source materials. The starting source material was rigorously purified by repeated sublimation via Physical Vapor transport (PVT) under high vacuum close to 10 4 Torr, as source Hg2X2 powder may contain impurities such as, oxides, carbon, sulphur, etc. About 500 g of a given source material is loaded into the feeding chamber of the pre-cleaned and outgassed Pyrex purification ampoule. Details of the purification and growth process can be found in references [10,18–20]. The crystal growth process was controlled by the advanced PID (proportional integral derivative) controllers with adaptive tuning mode and the precise ampoule transporter system which gradually changes the temperature of the ampoule by lowering or raising it inside the heating coil. The rate at which the ampoule can be lowered raised can be set as low as 1 mm per day for smooth temperature profile along the ampoule. Details of the crystal growing furnace can be found in Ref. [18]. The design of the growth ampoule was optimized for the self-seeding technique. The temperature of the growth ampoule was increased slowly to generate a seed for single crystal. The major difference in the growth procedures of three different mercurous halide crystals is the growth temperature. The growth temperature ranges of Hg2I2, Hg2Br2 and Hg2Cl2 are 200 °F–230 °F, 300 °F–330 °F and 370 °F–400 °F, respectively. ΔT, which is the temperature difference between the source and the crystal is a critical factor for high quality crystals. It was noticed that smaller ΔT makes the crystal grow slower but higher in quality whereas larger ΔT makes the crystals grow faster but lower in quality. Once the feeding Hg2X2 material was completely sublimed and crystalized, the crystal growing process is terminated with gradual decrease of the temperature inside the growth chamber. Due to single crystalline nature of the crystals, during cooling the grown crystals completely detach from the surrounding glass of the ampoule. Several crystals of each material (mercurous iodide, mercurous bromide and mercurous chloride) have been successfully grown. The diameter of the grown crystals is 48 mm, which is more than 3 times the diameter of the best quality crystals (13 mm) that has been reported so far [13]. The crystals are 70– 80 mm in height and 400–600 g in weight. The exact height and the weight of the grown crystals depend on the amount of the
11.8 Lattice parameter c
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Hg I
11.4
I
11.0
Br Cl
10.6 1.6
1.8 2 Ionic radii
2.2
Fig. 1. Ionic radii vs. lattice parameter c (a) and the atomic structure of the mercurous iodide (Hg2I2) unit cell.
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feeding material in the growth ampoules. XRD analysis were conducted on the naturally formed facet of the crystal as well as on the cut and polished surfaces of the grown crystals. 2.2. XRD analysis of the Hg2X2 crystals In order to determine the purity of the starting source material, XRD analysis was conducted by acquiring full 2θ–θ scan on the source material. To find out the efficiency of the purifying method,
Sample location 1 (Sublimate)
Sample location 2 (Black impurity)
Fig. 2. Schematic of the purifying ampoule showing the two different sample locations.
XRD analysis was performed on the sublimate after each purifying process. Samples from the sublimate were collected from the sample location 1 as shown in Fig. 2. In order to determine the type of impurity, the substance that remains in the feeding chamber of the purifying ampoule (sample location 2) was also collected for XRD analysis. Once the Hg2X2 crystals are fully grown, XRD analysis were performed to identify the orientation, lattice parameter c and the crystalline quality of the Hg2X2 crystals. For the analysis, a Rigaku high-resolution four-circle diffractometer with an 18-kW rotatinganode Cu source was used. In addition, high resolution x-ray diffraction (HRXRD) analysis were performed to determine the crystalline quality and the lattice parameter c of the grown crystals. During the performance of HRXRD, only the Cu Kα1 x-rays were selected using a monochromator that consists of two Ge (220) channel-cut crystals. In addition, a Ge (220) channel-cut crystal was used as the analyzer crystal. The diffractometer has a four-circle goniometer with X-, Y- and Z- translation stage in addition to ω, χ, and ϕ degrees of freedom. X-ray scans were collected from both [hk0] and [00l] reflections. In order to determine the crystalline quality, rocking curves of the [00l] reflections were
Fig. 3. Prepared 10 mm 12 mm 20 mm Hg2Br2 crystal (a), the fabricated AOM device (b) and the electrical circuit of the device (c). L1, L2 and L3 are inductors and C is a capacitor.
Fig. 4. Fabricated and tested gamma ray detector configurations consist of 12 mm 12 mm 4 mm, Hg2Br2, 12 mm 10 mm 3 mm Hg2I2 and 15 mm 15 mm 5 mm Hg2I2 substrate from left to right.
Fig. 5. Purifying ampoules before (a) and after (b) the first purifying process of Hg2I2.
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obtained from the naturally formed facet of the crystal. In addition, 2θ/ω scans were performed to determine the lattice parameter of the crystal. For device fabrication, the crystals need to be cleaved or cut and fine-polished. The XRD scans were also acquired on the polished facet and compared with the scans of the naturally formed facet to evaluate the efficiency of the polishing technique that was used. Furthermore, high resolution x-ray topographic images of the Hg2X2 crystals were taken using a double-crystal topography setup using Cu Kα1 radiation.
insulating layer, either chemically or via physical deposition methods. To evaluate detector performance, several different device configurations have been fabricated from planar to pseudohemispherical, to pixelated detectors at different thicknesses. Some of the device configurations and electrode contacts are shown in Fig. 4. The epoxy was used as both electrode and as a mean to connect the device to readout electronic circuitry using Au or Pt wires.
2.3. Acousto-optic device fabrication
3. Results and discussions
A PVT-grown Hg2Br2 crystal, size 15 mm 8 mm 3 mm, was cut and polished for fabrication of acousto-optic modulator (Fig. 3a). The optical beam is traveling along the (001) direction. The orientations of the crystal was confirmed using x-ray diffraction. Next, a pre-thinned transducer was mounted to the prepared Hg2Br2 using conductive epoxy, which also is acting as the bottom electrode. After the epoxy was cured and firmly bonded to the transducer and the substrate, a patch of epoxy was smeared on the transducer for the top electrode. Next, conductive Cu strips were attached for the electrical connections. Final part of the AO design involves electrical impedance matching network. Drawing of the electrical equivalent circuit for the fabricated device is shown in Fig. 3c. The final fabricated device for AOM application is shown in Fig. 3b.
3.1. Purification and crystal growth
2.4. Hg2X2 for gamma ray detection In preparation for gamma ray detection, samples were first cut and polished, followed by chemical etching to remove subsurface damages. Segmented pattern was created via photolithography. The inter-pixel or streets regions were passivated with an
Pictures of the purifying ampoules before and after the first purification process are shown in Fig. 5. After the first sublimation process, it was noticed that a black powdery substance (impurity) was remained in the feeding side of the ampoule. However, this substance was not further seen in any of the subsequent resublimation processes. XRD scans of the sample that was collected from the sample location 1 (sublimate) after the final purification process of Hg2I2 powder are shown in the Fig. 6. All the peaks observed corresponded to the Hg2I2 peaks as marked, indicating that the sublimate only consists of polycrystalline Hg2I2. Dark-colored substance was observed in the location no. 2 (Fig. 2) during purification as shown in Fig. 5. Literature survey indicates this material as Hg5Cl2O4 [10]. However, our XRD data did not match with the XRD pattern of Hg5Cl2O4. Thorough investigation on this residue is underway and finding will be published in an upcoming research article. The prominent cleavage planes of Hg2X2 crystals were found along (110) orientation. The crystals were then cleaved along (110) orientation and polished for XRD analysis. Photographs of cleaved and polished surfaces of the PVT-grown Hg2I2 crystal are shown in Fig. 7. XRD pattern collected along (110) orientation is shown in Fig. 8. The 2θ–θ full scan (Fig. 8-a) shows two sharp peaks that correspond to the (110) and (220) reflections. The results also displayed resolved kα1 and kα2 peaks, which can be seen only in high quality crystals. 3.2. High resolution X-Ray diffraction (HRXRD) analysis
Fig. 6. XRD pattern of the sublimate (sample location 1) after the final purification of Hg2I2.
Naturally-formed facets along (001) direction and (110) directions were observed on every PVT-grown Hg2X2 crystal. The facet along (001) direction of an Hg2I2 crystal was used in crystal quality analysis. The 2θ–ω scan and the ω scan of the naturally-formed facet of a Hg2I2 crystal from the (004) reflection was acquired using HRXRD. The measured full with at half maximum (FWHM) of the rocking curve of the Hg2I2 crystal was determined as 0.13°
Fig. 7. PVT-grown Hg2I2 crystal (a), cut and polished along the (001) plane (b), and the clean (110) cleavages (c).
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Fig. 8. XRD pattern collected along (110) orientation. (a) 2θ–θ scan (b) Resolve of kα1 and kα2 peaks on (220) reflection of Hg2I2 crystal.
(Fig. 9). In this figure, the inset is a picture of a naturally formed facet of the Hg2I2 crystal. To our knowledge, for bulk-grown single crystals, FWHM of 0.13° is the best that has been achieved so far on PVT-grown mercurous halide single crystals. The lattice parameter c of the grown Hg2I2 crystal that corresponds to 2θ at 30.77° was determined as 11.614 Å. Before making the AOM device, the cut or cleaved crystals need to be fine polished for optimal AOM performance. Smooth polishing of the bonding and the optical faces of the cut crystals is extremely important in acousto-optic applications. Cutting and rough polishing of the crystal may cause grains on the surface to become loose and reorient. In XRD these changes are observed as elevated FWHM of the rocking curve. The quality of the polished surface of the cut crystal was evaluated using HRXRD. Fig. 10 is the high resolution ω scan of a cleaved and fine polished Hg2I2 crystal from the (004) reflection. The measured full with at half maximum (FWHM) of the rocking curve was determined as 0.24°, which is very close to the measured FWHM (0.13°) of the naturally-formed
Fig. 9. High resolution rocking curve of the naturally-formed facet of Hg2I2 (004).
Fig. 10. High resolution ω scan of the cleaved and polished face of Hg2I2 crystal from the (004) reflection.
Fig. 11. Topographic image showing grain boundaries (A) and an array of threading screw dislocations (B) of the polished Hg2I2 (004) reflection.
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facet. The results not only confirm the excellent quality of the crystal, but also indicate that the polishing method has no adverse effects on the quality of the polished surfaces. The lattice parameter c that corresponds to the 2θ at 30.70° was determined as 11.639 Å, which is very close to the lattice parameter c (11.614 Å) that was obtained from the naturally-formed facet. Topographic image of the polished Hg2I2 (004) reflection is shown in Fig. 11. In this image random line dislocations are seen as the extended defects of the crystal. These features are very consistent with the features that can be observed in the XRD topograph (not included in this paper) that was taken from a naturallyformed facet of the Hg2I2 crystal. In some locations the dislocations are leading into formation of grain boundaries (A). An array of threading screw dislocations (B) is also visible on the image.
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3.3. AOM device testing Further testing was conducted on fabricated AOM device using 1152 nm and 1523 nm wavelength He-Ne laser beams. The first order diffraction signal of the device was clearly observed with diffraction angles of 1.2° and 2° for 1152 nm and 1523 nm laser beams, respectively. The diffraction images of the AOM with and without RF power is shown in Fig. 12. The magnitude of RF power was 1.5 W. The diffraction efficiencies of the 1152 nm laser polarizing at 45° to the acoustic wave and parallel to the acoustic wave were found to be 37% and 35%, respectively. The diffraction efficiencies of the 1523 nm wavelength laser polarizing parallel to the acoustic wave was found to be 28%.
Fig. 12. Diffraction images of the fabricated AOM (a) without the RF (b) with the RF power showing the first order diffraction.
Fig. 13. (a) Co-57 energy resolution of 2 mm Pseudo-hemispherical Hg2I2 detector at 122 keV was 1.86% FWHM. (b) Am-241 energy resolution of a 2 mm-thick Pseudo hemispherical Hg2I2 detector at 59.6 keV is 1.68% FWHM. The peaks are marked in purple.
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3.4. Gamma ray detector response To evaluate detector performance, Cobalt-57 (Co-57) spectral response was acquired using 2 mm-thick Pseudo-hemispherical Hg2I2 detector. The energy resolution of Co-57 at 122 keV was found to be 1.86% FWHM at room temperature. The energy resolution of Americium-241 (Am-241) response from a hemispherical Hg2I2 detector at 59.6 keV was found to be 1.7%. The results are shown in Fig. 13.
4. Conclusions High quality transparent single crystals of mercurous iodide (Hg2I2) of 48 mm in diameter and weighing 400–600 g were grown using unseeded PVT technique. The crystals grew without any contact with the side walls of the ampoule. The study shows the excellent crystalline quality of the grown crystals. X-ray full with at half maximum (FWHM) of the rocking curve of the natural facet was determined as 0.13°. The FWHM of the cleaved and fine polished facet was determined as 0.24°. Preliminary feasibility studies were conducted on AOM device fabrication using the grown crystals. The diffraction efficiencies with 1152 and 1523 nm wavelength lasers polarizing parallel to the acoustic wave were found to be 35% and 28%, respectively. The crystals were also used in fabricating devices for gamma ray and alpha particle detection. In this study, the energy resolutions as high as 1.86% FWHM at 122 keV and 1.7% FWHM at 59.6 keV were observed with Co-57 and Am-241, respectively at room temperature.
Acknowledgment The authors would like to thank Dave Mayers, Paul Deng, Helen
He and Julie Chen for their support on sample preparation. This research has been supported by the US Army Edgewood Chemical Biological Center, Contract number W911SR-14-C-0065 and US Army Research Laboratory, Contract numbers W911QX-06-C-0074 and W911QX-06-C-0074-P0006.
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